Descriptions

Surface and subsurface mapping are combined to determine the
geologic history along the San Gabriel fault near the town of Castaic.
Palomas Gneiss, Whitaker Granodiorite, and Pelona Schist are basement
terranes encountered in the subsurface. West of the San
Gabriel fault, basement is unconformably overlain by marine middle
to late Miocene Modelo Formation. The late Miocene to early Pliocene
Towsley Formation overlies the Modelo and was deposited in a
submarine fan environment. East of the San Gabriel fault, marine
Paleocene San Francisquito Formation accumulated while Pelona
Schist was undergoing regional metamorphism at depths of 20 to 27
kilometers. Nonmarine Oligocene (?) Vasquez Formation is faulted
against both the Pelona Schist and San Francisquito Formation.
Charlie Canyon Megabreccia accumulated in late Oligocene (?) time
as a large landslide deposit. The source is controversial, but may
have been from the LaPanza Range. Pelona Schist-bearing San Francisquito
Canyon Breccia accumulated in late Miocene (?) (Barstovian)
tune as the Pelona Schist first became subject to erosion in northern
Soledad basin. Nonmarine alluvial fan and lacustrine deposits of the
middle to late Miocene Mint Canyon Formation unconformably overlie
older units east of the San Gabriel fault and apparently intertongue
with San Francisquito Canyon Breccia A late Miocene (Mohnian)
marine transgression resulted in deposition of the Castaic Formation
in Soledad basin while the Modelo and possibly Towsley Formations
accumulated in Ventura basin. Violin Breccia intertongues with
Castaic Formation and accumulated at the base of a Miocene San
Gabriel fault scarp Marine Pliocene Pico Formation unconformably
overlies the Castaic Formation east of the San Gabriel fault and conformably
overlies the Towsley Formation west of the San Gabriel
fault; it is overlain conformably by marine to nonmarine Plio-
Pleistocene Saugus Formation. Quaternary landslides, and older,
intermediate, and younger alluvium overlie older rocks throughout
the study area.
The St. Francis fault may have been active in late Oligocene
and early Miocene time, removing Charlie Canyon Megabreccia by
right-slip from a possible source in the La Panza Range. The low-angle
San Francisquito fault and associated Bee Canyon fault and
San Francisquito syncline were active after deposition of the Charlie Canyon Megabreccia but prior to deposition of the Mint Canyon
Formation The San Gabriel fault became active after deposition
of the Mint Canyon Formation in late Miocene time The San Gabriel
fault exhibits right-lateral separation of approximately 60 kilometers
on the Mint Canyon and older formations, approximately 30 kilometers
on Violin Breccia and Modelo Formation, and about two kilometers on
the Pico Formation. San Gabriel fault "C" (?) and Castaic Hills
reverse fault moved in late Miocene time and became inactive prior
to deposition of the Pico Formation. The Charlie Canyon anticline,
Charlie Canyon syncline, Castaic anticline, and Ridge Basin syncline
were formed in the Castaic Formation prior to deposition of the Saugus
Formation The Saugus is deformed by (1) folding which produced
the Dry Canyon syncline, Dry Canyon anticline, Townsend syncline,
Loma Verde anticline, North and South Hasley Canyon synclines, and
Oak Canyon anticline, (2) reverse faulting on the south-dipping Hasley
fault; (3) normal faulting in eastern Castaic Hills oil field; (4) normal
faulting on the San Gabriel fault. The San Gabriel fault may also
offset Quaternary alluvium by normal separation in Castaic Creek.
Geomorphic evidence suggests continuing movement on the San Gabriel
fault, but the nature of seismic activity on the fault is still unresolved.